Electrically powered vehicles capable of running with electric energy, such as hybrid vehicles, electric vehicles and fuel cell vehicles, have been developed and put into practical use as environmentally friendly vehicles. An electric-powered vehicle has mounted thereon a motor generating vehicle drive force and a power storage device storing electric power for driving the motor.
Further, some electric-powered vehicles have mounted thereon a mechanism generating charge electric power for the power storage device during running. For example, a hybrid vehicle has mounted thereon the above-described internal combustion engine in addition to the above-described motor and the above-described power storage device. Outputs from the internal combustion engine are used as vehicle drive force and/or generated motive power for charging the power storage device.
In a conventional hybrid vehicle, charge and discharge control of maintaining SOC (State Of Charge) indicating a remaining capacity of the power storage device at a level of 50 to 60% of full charge is generally performed such that regenerative electric power during regenerative braking can be accepted while ensuring electric power for generation of drive force by the motor.
On the other hand, in recent years, a structure in which an on-vehicle power storage device of a hybrid vehicle is charged by a power supply outside the vehicle (hereinafter also called an “external power supply”) has been proposed. In an electric-powered vehicle chargeable by an external power supply, in order to improve energy efficiency, such charge and discharge control that the power storage device is charged to a full charge level by external charging before the start of driving, while stored electric energy is used up almost to a lower limit value of SOC at the end of driving is targeted.
For this reason, a hybrid vehicle has also been developed in which, in addition to a conventional running mode of maintaining SOC of the power storage device, i.e., stored electric energy, at a constant level, a running mode of mainly running only with a motor without maintaining SOC of the power storage device is newly introduced. It is noted that, the former running mode is referred to as an “HV (Hybrid Vehicle) mode” since the engine is operated for generating charge electric power for the power storage device, or a “CS (Charge Sustaining) mode” (in the present specification, hereinafter referred to as a “CS mode”) since stored electric energy charged in the power storage device is maintained. The latter running mode is referred to as an “EV (Electric Vehicle) mode” since the vehicle mainly runs only with the motor, or a “CD (Charge Depleting) mode” (in the present specification, hereinafter referred to as a “CD mode”) since stored electric energy decreases.
Regarding a power supply system mounted on such a hybrid vehicle, Japanese Patent Laying-Open No. 2008-109840 (PTL 1), for example, discloses a structure in which a plurality of power storage devices (batteries) are connected in parallel so as to increase the distance that can be traveled by stored electric energy in the on-vehicle power storage devices.
In the structure disclosed in PTL 1, an acceptable charge amount and an available discharge amount are calculated for each power storage device in accordance with a remaining capacity, and a charge allocation ratio and a discharge allocation ratio among the plurality of power storage devices are determined based on the calculated acceptable charge amount and available discharge amount. Since charge and discharge of each power storage device is controlled in accordance with the determined allocation ratios, performance of the system can be exhibited to its maximum, even when charge and discharge characteristics of the plurality of power storage devices are different among them.
Moreover, Japanese Patent Laying-Open No. 2008-167620 (PTL 2) discloses a structure of a power supply device in a vehicle having mounted thereon a main power storage device and a plurality of sub power storage devices, wherein a converter corresponding to the main power storage device and a converter shared by the plurality of sub power storage devices are provided. According to this structure, energy that can be stored can be increased while reducing the number of converters.
In the structure disclosed in PTL 2, one of the plurality of sub power storage devices is selectively connected to the converter, and drive electric power for a vehicle drive motor is supplied from the main power storage device and the selected sub power storage device. In such a power supply device, the plurality of sub power storage devices are used sequentially such that a new sub power storage device is connected to the converter when SOC of a sub power storage device that is being used decreases, thereby increasing the running distance with stored electric energy (EV running distance).